After crossing the homozygous parents and generating a heterozygous hybrid plant (F 1 ),
Mendel would allow the hybrid plant to self-fertilize. In the subsequent F 2 plants orF 2
generation, plants with the recessive trait would reappear (Fig. 2.5b). Mendel realized
that the recessive allele was not replaced or destroyed by the dominant allele, but its phe-
notype was just masked in the heterozygous individuals. With his intricate recordkeeping of
counting the plants with different phenotypes, Mendel observed that the dominant plants
occurred in 75% of individual F 2 plants, while recessive plants occurred at a frequency
of 25%. Mendel’s crosses may be visualized in a graphical table called aPunnett square
that depicts the number and variety of genetic combinations in a genetic cross. The latter
was named after Reginald Punnett, who worked with William Bateson to confirm exper-
imentally the findings of Gregor Mendel. Their investigations of the exceptions to
Mendel’s rules led to the discovery of genetic linkage in the pea, discussed later in this
chapter. Using a Punnett square, the possible genotypes of the gametes from each parent
are placed on adjacent axes, and the matrix within the Punnett square represents all possible
outcomes from sexual reproduction.
Using his crossing data, Mendel realized that plants contained two copies of genetic
material. Although he did not know that each plant had two different sequences of DNA
on the two homologous chromosomes, he could predict the expected segregation frequen-
cies over all the traits that he tracked over multiple generations. The fundamental process
that Mendel discovered was that plants contained two versions of every gene, and that
those genes were discrete particles that could separate from one another over the
generations.
2.2.1 Law of Segregation
In his crosses using single traits, ormonohybrid crosses, Mendel described the first of his
genetic laws explaining how traits are passed between generations. He didn’t know that
DNA was controlling the traits he was observing, but we will state his law on the basis
of current knowledge that DNA is genetic material and is stored in chromosomes.
Because dominant and recessive alleles segregate from one another in progeny derived
from heterozygous plants, he described thelaw of segregation, which states that two homo-
logous chromosomes separate from one another during the production of sex cells. In prac-
tical terms, this means that half of the sex cells will be produced with one allele and half
with the other allele in a heterozygous plant.
2.2.2 Law of Independent Assortment
Mendel also crossed plants that differed at multiple traits at the same time. When plants that
differed at two traits were crossed, or weredihybrid crosses, Mendel determined that the
traits segregated independently from one another (Fig. 2.6). This phenomenon was
described in thelaw of independent assortment, where chromosomes from different homo-
logous chromosome pairs separate independently from one another during the production
of sex cells. Chromosomes are independent molecules of DNA, and only homologous
chromosomes pair with one another during gamete production. Therefore, nonhomologous
chromosomes will divide completely randomly into the daughter cells.
It is an interesting historical fact that the traits that Mendel studied were controlled by
genes on different chromosomes. This is often deemphasized when discussing Mendel’s
work and it should not be, because if the genes had been on the same chromosome, his
28 MENDELIAN GENETICS AND PLANT REPRODUCTION